In general, a step attenuator is used as a variable attenuator required for an analog circuit. In this case, the step attenuator is composed of a plurality (n.gtoreq.1) of series-connected unit step attenuators and classified into two types of .pi. and T.
FIG. 9 shows an example of prior art .pi.-type variable attenuator, in which three .pi.-type unit step attenuators 4.sub.1, 4.sub.2 and 4.sub.3 are connected in series. Each unit step attenuator 4.sub.i (i=1, 2 and 3) is composed of three resistors 6.sub.i, 7.sub.i and 8.sub.i, a switch FET (Field Effect Transistor) 5.sub.i connected in parallel to the resistor 6.sub.i, a high resistance resistor 3.sub.i connected to a gate of the switch FET 5.sub.i, a transfer gate 9.sub.i connected in series to the resistor 7.sub.i and a transfer gate 10.sub.i connected in series to the resistor 8.sub.i.
The attenuation rate of each unit step attenuator 4.sub.i can be decided on the basis of three resistors 6.sub.i, 7.sub.i and 8.sub.i and the on/off resistance of the switch FET 5.sub.i. Further, when operated as the attenuator, the switch FET 5.sub.i is turned off in response to a control signal inputted to the gate thereof, and the two transfer gates 9.sub.i and 10.sub.i are turned on in response to control signals 13.sub.i and 14.sub.i, respectively. Further, when a RF (radio frequency) signal inputted to the input terminal 90 is only passed therethrough (without operated as the attenuator), the switch FET 5.sub.i is turned off and the transfer gates 9.sub.i and 10.sub.i are also turned off. Therefore, when assumption is made that the attenuation rates of the unit step attenuators 4.sub.1, 4.sub.2 and 4.sub.3 are 4 dB, 8 dB and 16 dB, respectively, if only the unit step attenuator 4.sub.1 is operated as the attenuator and other two unit step attenuators 4.sub.2 and 4.sub.3 are not operated as the attenuators, the RF signal inputted to the input terminal 90 is attenuated by 4 dB at an output terminal 100. Further, when only the step attenuators 4.sub.1 and 4.sub.2 are operated as the attenuators, the attenuation rate is 12 dB (=4 dB+8 dB); when only the step attenuators 4.sub.1 and 4.sub.3 are operated as the attenuators, the attenuation rate is 20 dB (=4 dB+16 dB); and when all the step attenuators 4.sub.1, 4.sub.2 and 4.sub.3 are operated as the attenuators, the attenuation rate is 28 dB (=4 dB+8 dB+16 dB).
As described above, it is possible to obtain various attenuation rates of 4, 8, 12, 16, 20, 24 and 28 dB, respectively by combining the three unit step attenuators appropriately.
Further, in FIG. 9, control signals 2.sub.1, 2.sub.2 and 2.sub.3 ; 13.sub.1, 13.sub.2 and 13.sub.3 ; and 14.sub.1, 14.sub.2 and 14.sub.3 are applied by a control section (not shown), and the capacitors 1 and 30 are of dc cut-off capacitor.
Further, FIG. 10 shows an example of prior art T-type variable attenuator, in which four T-type unit step attenuators 60.sub.i, 60.sub.2, 60.sub.3, and 60.sub.4 are connected in series. Each unit step attenuator 60.sub.i (i=1, 2, 3 and 4) is composed a switch FET 62.sub.i, a high resistance resistor 68.sub.i connected to the gate of the switch FET 62.sub.i, a transfer gate 65.sub.i, three resistors 63.sub.i, 64.sub.i and 66.sub.i arranged into T-shape via the transfer gate 65.sub.i, and a resistor 67.sub.i connected to the gate of the transfer gate 65.sub.i. Further, a series circuit of the two resistors 63.sub.i and 64.sub.i is connected in parallel to the switch FET 62.sub.i.
In this T-type attenuator, it is possible to change the attenuation rate by combining the four unit step attenuators 60.sub.i, 60.sub.2, 60.sub.3 and 60.sub.4 appropriately.
Further, the reason why a high resistance resistor 68.sub.i is connected to the gate of the switch FET 62.sub.i in FIG. 10 (or a high resistance resistor 3.sub.i is connected to the gate of the switch FET 5.sub.i in FIG. 9) is to reduce the gate current of the FET (because the FET is of voltage drive type) and to suppress the influence of the control section.
In the prior art attenuator, as described above, the attenuation rate can be determined by combining a plurality of unit step attenuators, because the attenuation rate thereof can be decided on the basis of the attenuation resistor and the on/off resistance of the switch FET connected in parallel to the attenuation resistor. As a result, when a plurality of attenuation rates are required, the number of unit step attenuators, that is, the number of switch FETs must be increased with increasing number of sorts of attenuation rates. On the other hand, when a signal transmission loss is defined as a loss determined when an RF signal is passed through the attenuator device under the condition that the variable attenuator is not operated (i.e., the switch FETs are kept turned on), since the transmission loss is caused by the turn-on resistance of the switch FETs, there exists a problem in that the larger the number of the required attenuation rates is, the larger will be the signal transmission loss.
To overcome this problem, it is possible to increase the gate width of the switch FET so that the turn-on resistance can be reduced when the RF signal is transmitted from the drain to the source of the switch FET. In this case, however, when the gate width of the FET increases, since the layout area thereof increases and further the parasitic capacitance of the FET also increases, another problem arises in that the frequency characteristics deteriorate.
Further, when the attenuation rate error of the unit step attenuator results from the dispersion (difference) of the switch FET characteristics caused during the manufacturing process, when the number of the unit step attenuators increases, since the maximum attenuation rate error obtained by adding all the attenuation rate errors of all the unit step attenuators also increases, there exists the other problem in that the attenuation rate selectable by the variable attenuator also differs. For instance, in the case of the example shown in FIG. 9, if the attenuation error (dispersion) of each unit step attenuator 4.sub.i is assumed to be .+-.0.5 dB, the attenuation error is .+-.0.5 dB when the attenuation rate of the variable attenuators are 4, 8, and 16 dB respectively; .+-.1.0 dB when the attenuation rate thereof are 12, 20, and 24 dB respectively; and .+-.1.5 dB when the attenuation rate thereof is 28 dB.
Therefore, the system design has been so far complicated by the non-linear characteristics of the respective attenuation rates of the respective unit step attenuators. On the other hand, in order to overcome the problem related to the non-uniformity of the attenuation rate error, it may be possible to increase the number of unit step attenuators or to provide a new unit step attenuator having the maximum attenuation rate (e.g., 28 dB). In this case, however, there arises another problem in that the layout area increases and thereby the signal transmission loss increases, as already explained.